It's such a taboo topic that some doctors and nurses are uncomfortable talking about it.
But it is practised by both straight and gay couples, and children, and has huge health risks, especially when it is shrouded in silence.
Anal sex is surrounded by misconceptions. Some people wrongly believe it is a form of safe sex.
But a person is 18 times more likely to contract HIV during unprotected anal sex than during unprotected vaginal sex, according to Glenn de Swardt, director of Health4men clinics, for men who have sex with men.
Anal sex is not mentioned in safe-sex and HIV campaigns. As a result, health activists have found that people incorrectly perceive it to be safer than vaginal sex.
University of Cape Town doctoral researcher Zoe Duby said: "[My] research suggests that people practise anal sex as a form of safe sex, thinking they cannot get HIV from it because they have not heard about it in safe-sex messages. A substantial proportion of heterosexual couples practise anal sex – a greater number than homosexual men."
University of Pretoria and sexologist Elna McIntosh said: "It is an absolute taboo but there is so much education that needs to be done. For the past 10 years I've known that schoolchildren do it to protect a girl's virginity. No wonder we have such high HIV rates in the 15 to 25 years group." She said teenagers were more worried about pregnancy than HIV or the human papilloma virus, which causes cervical, throat and anal cancer.
McIntosh has worked in youth radio and runs the Intersexions Facebook page. She said young people wanted to ask about anal sex.
"No one is talking about it," said De Swardt. "One cannot separate anal sex from the fact that people view it as shameful, dirty and wrong," he said.